Plastics have many strong points, such as being lightweight and strong, capable of transparency and non-permeability to water or gases, easily colored and shaped, and due to their high usability, their production volumes and usage volumes are also increasing. However, accompanying this, the volumes of discarded plastics have been increasing, and the increased burden on the environment has become a social problem. To solve these problems and also to use limited resources more effectively, there has been intensive research in recent years on methods of recycling plastics.
Methods of recycling plastics include, for example: material recycling, in which plastics are repelleted and used as raw material for new molded products; thermal recycling, in which plastics are combusted and the thermal energy recovered; and chemical recycling, in which plastics are thermally decomposed for use as reduction agents for blast furnaces, or decomposed down to fats/oils or monomers and reused as raw materials for plastics.
Among these, material recycling can be said to be the most desirable method, as it requires only a small amount of energy for recycling. However, in order to prevent the recycled plastics from having reduced physical properties, it is necessary to prevent the mixture of different plastic types when the plastics are repelleted. Therefore, there is a need for a highly accurate method of identifying plastics.
Conventionally, the main methods used for identifying plastic types are spectroscopic methods such as infrared spectroscopy using near infrared light. However, with these spectroscopic methods, identification is difficult when the plastics to be identified are dark-coloured plastics (for example, the plastics used for such items as the chassis of television receivers and many other household electrical appliances are colored black), because of reasons such as the applied near infrared light being absorbed, and it is therefore difficult to obtain infrared absorption spectra.
Furthermore, when identifying plastic types using obtained infrared absorption spectra, it is usual to compare and match the obtained infrared absorption spectra with a group of infrared absorption spectra of various standard plastics. The process of comparing and matching involves searching for the infrared absorption spectrum of a standard plastic that is the best fit with the obtained infrared absorption spectrum, and thereby identifying the type of plastic.
Very often plastics that are to be recycled contain flame retardants and other additives. However, the standard plastics that have been compared and matched are basically “pure” polymers, and plastics that contain additives have not been included as objects of comparison and matching. Furthermore, the data for the infrared absorption spectra of standard plastics usually have been data obtained by penetrating infrared light through the items to be identified (penetration method).